1. Field of the invention
The present invention provides a method and system for measuring electron densities in a plasma processing system, such as is used in semiconductor processing systems.
2. Description of the Background
There are at least three known microwave-based techniques for determining plasma electron densities: (1) microwave interferometry, (2) measurement of reflection and absorption, and (3) perturbation of cavity resonant frequencies. Each of the concepts is described in simplified terms below.
Microwave interferometry involves the determination of the phase difference between two microwave beams. The first bears provides a reference signal, and the second beam passes through a reactive environment and undergoes a phase shift relative to the first beam. The index of refraction is calculated from the measured change in the phase difference between the two beams. This interferometric technique has been documented by Professor L. Goldstein of the University of Illinois at Urbana. Interferometry is described in the following U.S. Pat. Nos. 2,971,153; 3,265,967; 3,388,327; 3,416,077; 3,439,266; 3,474,336; 3,490,037; 3,509,452; and 3,956,695, each of which is incorporated herein by reference. Some plasma properties may be indirectly determined from measurements of the absorption of a microwave beam as it traverses a region in which a plasma is present. Signal reflections in plasmas are described in U.S. Pat. No. 3,599,089 and 3,383,509.
Plasma electron densities have also been measured using a technique which measures the perturbations of cavity resonant frequencies. The presence of a plasma within a resonator affects the frequency of each resonant mode because the plasma has an effective dielectric constant that depends on plasma electron density. This technique has been documented by Professor S. C. Brown of the Massachusetts Institute of Technology. Portions of this technique are described in U.S. Pat. No. 3,952,246 and in the following non-patent articles: Haverlag, M., et al., J Appl Phys 70 (7) 3472-80 (1991): Measurements of negative ion densities in 13.56 MHZ RF plasma of CF4, C2F6, CHF3, and C3F8 using microwave resonance and the photodetachment effect and Haverlag, M., et al., Materials Science Forum, vol. 140-142, 235-54 (1993): Negatively charged particles in fluorocarbon RF etch plasma: Density measurements using microwave resonance and the photodetachment effect.
It is an object of the present invention to provide a more accurate plasma measuring system than the prior art.
It is a further object of the present invention to provide an improved plasma measuring system using plasma induced changes in the frequency of a microwave oscillator.
These and other objects of the present invention are achieved through the use of a feedback loop that measures and controls a signal passing through a plasma chamber. Using a computer or digital signal processor (DSP), the present invention measures a frequency of a signal passing through the plasma and compares the measured frequency to a desired frequency. Based on a difference between the measured and desired frequencies, the computer/DSP controls the plasma generator to increase or decrease power to the plasma chamber, thereby affecting plasma electron density.